a. Field of the Invention
The instant invention is directed toward virtual-electrode catheters and to methods for using such virtual-electrode catheters. More specifically, it relates to bipolar and multipolar, virtual-electrode catheters having at least one internal electrode and at least one surface electrode, and to methods of using these catheters for treatment of cardiac arrhythmias via, for example, radiofrequency (RF) ablation.
b. Background Art
Conventional catheter techniques of RF ablation for treating cardiac arrhythmias use RF electrodes in a unipolar mode. In this mode, only the active RF electrodes are placed at the site of the ablation. The dispersive electrodes are placed at locations remote from the ablation site, typically on the skin of the patient.
In the unipolar mode, the RF current decreases as 1/r2, and the RF energy decreases as 1/r4, where “r” is the radial distance from an active electrode of a catheter. Because tissue ablation is caused by RF energy deposition into the tissue, the depth of the ablation is limited to a narrow rim around the catheter electrode. Increased lesion depth, therefore, requires high power. High power, however, causes rapid temperature increases and potential “hot-spots” at the electrode-tissue interface.
The virtual electrode technique mitigates this problem of temperature increases at the electrode-tissue interface by using cooled conductive fluid flowing onto the tissue surface. The fluid flow rate necessary to provide adequate surface cooling depends upon the RF power being delivered. The higher the power, the higher the flow rate that is necessary. To create lesions 3-4 mm deep using existing devices may require 50 watts for 60 seconds and a fluid flow rate of 72 ml per minute. For a full-circumferential lesion, these same existing devices require a minimum of two separate procedures at these settings. The minimum RF energy delivered during the 120 seconds total duration is, therefore, 6000 Joules; and the total volume of fluid delivered is over 140 ml. By contrast, for a typical pulmonary vein of 22 mm diameter, a lesion size of 60 mm×3 mm×3 mm obtained with a temperature rise of 50° C. requires a total energy of only about 120 Joules. This means that only 2% of the applied RF energy is used to create the lesion. The remaining 98% of the applied energy is lost to heating other substances such as the infused fluid, the catheter body, surrounding tissue, blood, and other tissue fluids. Existing techniques can be, therefore, highly inefficient.